1. Field of the Invention
The present invention relates to a method which allows selective recovery of americium from a nitric aqueous phase containing americium, curium and fission products including lanthanides, but which is free of uranium, plutonium and neptunium and which only contains these three last elements in trace amounts.
This method finds applications in the field of processing and recycling irradiated nuclear fuels where it has a most particular advantage for removing americium from raffinates stemming from processes for extracting and purifying uranium and plutonium such as the PUREX and COEX™ methods.
2. Description of the Related Art
The methods, which allow extraction and purification of uranium and plutonium present in liquors for dissolving irradiated nuclear fuels like the PUREX method (which is presently used in factories for processing irradiated nuclear fuels) and the COEX™ method (which is described in the international PCT application WO 2007/135178, [1]), generate effluents to which the name of raffinates is given.
These raffinates are aqueous solutions with strong nitric acidity, typically from 2 to 5 M, which contain americium, curium, lanthanides such as lanthanum, cerium, praseodymium, neodymium, samarium and europium, fission products other than lanthanides such as molybdenum, zirconium, rubidium, ruthenium, rhodium, palladium, and yttrium, as well as other metal elements which are neither lanthanides nor fission products, such as iron and chromium.
Americium is the main contributor to residual radioactivity after 300 years of waste stemming from the packaging of these raffinates (notably by vitrification). As an illustration, the time required for this radioactivity to return to a level of the same order as that of natural uranium used for making nuclear fuels is about ten thousand years.
Selective recovery of the americium present in raffinates stemming from the PUREX and COEX™ methods, before the latter are sent to vitrification, would allow significant reduction in the thermal load of the vitrified waste and consequently in the storage footprint of this waste.
In the irradiated fuel, after 5 years of cooling, 90% of curium is in the form of Cm-244, having a radioactive period of 18 years. The other isotopes Cm-243, Cm-245, Cm-246, with a period of more than 30 years only represent 10% of the isotopy of curium. As curium is a neutron emitter, making its handling delicate, and since Cm-244 leads to Pu-239 by decay with a period of less than 30 years, it proves to be more advantageous not to directly recycle curium with americium.
In order to recover americium from an aqueous phase, a certain number of methods have been suggested.
The SESAME method is notably one of them.
This method, which is described in French patent No. 2 731 717, [2], provides oxidation of americium via an electrochemical route—which is stable in an aqueous solution with a degree of oxidation III—into americium(VI) by using an electrochemical mediator, the role of which is to promote formation of americium(VI) on the one hand and a complexing agent on the other hand, the role of which is to stabilize americium(VI) in an aqueous solution. Hexavalent americium may then be selectively extracted from the aqueous phase, in which it is found, with an organic phase containing a solvating extractant such as tri-n-butyl phosphate (or TBP), or an acid extractant such as bis-2-ethylhexyl phosphoric acid (or HDEHP).
The use of the SESAME method for selectively recovering americium from raffinates stemming from the PUREX and COEX™ methods cannot be contemplated for the simple reason that the presence of oxidizable fission products significantly alters the electrochemical oxidation yield of the americium and therefore the yield for extracting this element with the organic phase. This is moreover the reason for which this method was suggested and used up to now for treating aqueous solutions only containing americium, curium and possibly lanthanides like those stemming from the SANEX and DIAMEX methods.
Moreover, there is the TALSPEAK method.
This method, which is described by B. Weaver and F. A. Kappelmann in the Report ORNL-3559, 1964, [3], makes provision for achieving an americium-curium partition after having separated both elements from the lanthanides and other fission products. To do this, it uses an organic phase consisting of a TBP/HDEHP mixture in an aromatic diluent of the toluene or di-isopropylbenzene type, and an aqueous phase containing a polyaminocarboxylic acid/hydroxycarboxylic acid mixture and the pH of which is adjusted to a value from 2 to 3 by adding a base.
Applied to the processing of a raffinate stemming from a PUREX or COEX™ method, the TALSPEAK method would require reduction in the acidity of this raffinate in order to return its pH to a value above 1 with, as a result, a non-negligible risk of hydrolysis of the fission products present in the raffinate, which may lead to the formation of insoluble products, for example containing zirconium, molybdenum or ruthenium.
Moreover, in the TALSPEAK method, the separation of americium and of curium from the fission products can only be obtained with an organic phase comprising an aromatic diluent, while the PUREX and COEX™ methods use an aliphatic diluent.
The result of this is that the application of the TALSPEAK method downstream from a PUREX or COEX™ method can no longer be contemplated.
Further there is the ALINA method.
This method, which is described in patent application US No. 2007/0009410, [4], provides extraction of americium from an aqueous phase containing curium and possibly other elements such as lanthanides, by using an organic phase consisting of a synergistic (chlorophenyl) acid/tri(ethylhexyl)phosphate or (chlorophenyl)dithio-phosphinic acid/trialkylphosphinic oxide mixture in i-octane and t-butylbenzene. The selectivity of this mixture would be sufficient for selectively extracting americium from an aqueous phase containing from 0.1 to 0.5 mol/L of nitric acid.
The application of this method to the processing of a raffinate stemming from a PUREX or COEX™ method would require, as in the case of the TALSPEAK method, reduction of the nitric acid content of this raffinate so as to bring it back to a maximum value of 0.5 mol/L if the intention is to obtain sufficient extraction and separation performances, with, there again, a risk of hydrolysis of the fission products present in the raffinate and therefore of formation of insoluble products.
Moreover, the organic phase used in the ALINA method may strongly extract certain fission products such as zirconium, molybdenum, ruthenium and palladium, which has the effect of reducing the loading capacity and the extracting power of this phase with regard to americium and of notably complicating the method if the intention is to then recover the americium alone in a suitable aqueous phase.
Further, the organic phase used in the ALINA method comprises four constituents, one of which leads to degradation products containing sulfur or chlorine atoms, which are difficult to handle in an industrial process in the nuclear field because of the risk of formation of sulfuric or hydrochloric acid. The presence of these four constituents considerably complicates the processing of the degraded organic phase which moreover has not been the subject of studies to this day.
Further, there is the DIAMEX 2 method.
This method described in [5], makes provision for extracting the americium from an aqueous phase containing curium by using an organic phase consisting of DMDOHEMA malonamide in TPH. The selectivity of this molecule would be sufficient in at least 1.7 mol/L nitric medium, for extracting more than 99% of the americium while leaving 99% of the curium in the aqueous phase, according to a test applied on a reformed solution in 2002. If this method has the advantage of being able to process a strongly acid solution, it requires downstream extraction cycles with other organic solutions, in order to obtain in fine an aqueous solution only containing americium and curium, which complicates the overall picture.
Finally there is the DIAMEX-SANEX method.
This method was proposed in French patent No. 2 845 616, [6] for separating actinides with a degree of oxidation of III, i.e. americium and curium, from lanthanides downstream from a PUREX method.
This method is based on the use of two extractants operating in separate chemical domains, i.e.: a first extractant of the solvating type, which is capable of extracting together the actinides(III) and lanthanides from an aqueous phase with strong nitric acidity, and a second extractant of the acid type, which is capable of extracting lanthanides from an aqueous phase with low nitric acidity.
This method schematically consists of co-extracting the actinides(III) and lanthanides from the aqueous phase, a strongly acid phase, in which they are found, by means of an organic phase containing the solvating extractant and then, after having added the acid extractant to the organic phase, of de-extracting the actinides(III) from this organic phase by means of a weakly acid aqueous solution. The acid extractant then allows the lanthanides to be retained in the organic phase and to thereby avoid that the latter are de-extracted together with the actinides(III).
Now, it turns out that within the scope of their work, the Inventors surprisingly noticed that the use of two extractants operating in separate chemical domains may also be exploited for selectively recovering americium from a nitric aqueous phase containing, in addition to this element, curium and fission products including lanthanides, provided that the first extractant is suitably selected on the one hand and that certain of the operating conditions under which this use is made, are suitably selected on the other hand.
And it is on this observation that the present invention is based.